1. Field of the Invention
The invention relates to a circuit arrangement and a method for generating and amplifying an amplitude-limited DC signal whose level voltage is substantially proportional to the logarithm of the voltage amplitude of an input signal. Generally, the invention relates to the field of analog integrated circuits.
2. Description of the Related Art
Such circuit arrangements as well as such methods fulfill two functions. On the one hand, they are used for amplifying an AC signal which is applied to the circuit arrangement via its input terminals and, on the other hand, the amplitude of the output signal is limited by such a circuit arrangement and such a method. Consequently, the word xe2x80x9climiter amplifierxe2x80x9d is generally used for such a circuit arrangement. Such limiter amplifiers are used, for example, in the frequency-modulated IF amplification in radio receivers or for gaining carriers from amplitude-modulated signals.
When such a limiter amplifier is given a simple extension, for example, in the form of a rectifier stage, a DC voltage can be generated which is widely proportional to the logarithm of the amplitude of the voltage of the input signal. When the amplitude of the output signal is plotted against the logarithm of the amplitude of the input voltage, a wide range of the input voltage shows a straight characteristic with a substantially constant rise. This straight characteristic is commonly referred to as xe2x80x9clevel characteristicxe2x80x9d.
Regarding such circuit arrangements and such methods for generating and amplifying an amplitude-limited DC signal, it is to be taken into account that the level voltage of the DC signal in the case of a frequency-modulated input signal (referred to as xe2x80x9cFM-modulated input signalxe2x80x9d) is distorted when the input signal has previously passed through a circuit having a frequency-dependent amplitude variation, for example, in the form of an IF filter. In such a circuit, the frequency modulation (FM) is converted into an amplitude modulation (AM) which then undoubtedly becomes manifest as an unwanted level fluctuation in the level voltage of the DC signal, although the input signal was basically only frequency modulated (FM).
Based on the previously mentioned drawbacks and shortcomings, it is an object of the present invention to provide a circuit arrangement and a method of the type described in the opening paragraph, by which the level indication, i.e. the level voltage of the DC signal generated and amplified in the circuit arrangement and by the method only depends on the amplitude and not on the frequency of the input signal.
This object is solved by a circuit arrangement as defined in claim 1 and by a method as defined in claim 18. Advantageous embodiments and essential further improvements of the present invention are defined in the dependent claims.
In accordance with the teaching of the present invention, the level signal is corrected by means of a correction signal gained from the frequency (FM) demodulator unit, which correction signal compensates the influence of the frequency-dependent amplitude variation on the level signal. In other words, the amplitude-limited DC signal generated and amplified in accordance with the invention and having a level voltage which is substantially proportional to the logarithm of the voltage amplitude of an input signal only comprises information about the amplitude and not about the frequency.
It will be appreciated by those skilled in the art that the present invention provides a correcting compensation of the influence of the frequency modulation on the (logarithmic) level voltage. Such a compensation is required because frequency-modulated signals have amplitude fluctuations which were absent before they passed through a filter stage, because the filter stage has a frequency-dependent passband attenuation. By virtue of the present invention, it is surprisingly possible for those skilled in the art to acquire considerably more accurate indications of the level voltage about the signal level so that the circuit arrangement can be used in completely new fields of application.
The fundamental aspect of the present invention is that the level information of the input signal is initially indicated and/or detected by means of the at least one amplifier circuit comprising the at least two amplifier stages. After the output signal of the last amplifier stage, particularly the collector circuits of the last amplifier stage, has been frequency-demodulated by means of the at least one frequency (FM) demodulator unit, the output signal of the frequency (FM) demodulator unit, particularly the AC component of the output signal of the frequency (FM) demodulator unit, is corrected in the at least one quadrature stage. Finally, the level voltage of the output signal of the quadrature stage is added to the level voltage of the amplifier stages in at least one current adder unit.
In accordance with a preferred embodiment of the present invention, the quadrature stage is constituted by at least one, particularly non-negatively fed-back differential amplifier stage, at least one intermediate stage assigned to the differential amplifier stage, and at least one subsequent multiplier stage for multiplying the output signal of the intermediate stage. This multiplication product is added in the at least one current adder unit to the parts of the level characteristics of the amplifier stages.
In this connection, the level voltage of the output signal of the quadrature stage can be adapted to different circuits having a frequency-dependent amplitude variation and preceding the amplifier circuit, for example, to IF filters each having differently curved filter response curves, by varying the amplification of the quadrature stage.
The differential amplifier stage is suitably connected to the output of the frequency demodulator stage. In this connection, the differential amplifier is principally a linearly amplifying DC amplifier whose output voltage is proportional to the difference between the two input voltages. The basic circuit comprises two transistors jointly connected to the emitter and fed from a common constant current source. The differential amplifier is usually symmetrical because temperature influences and non-linearities then substantially compensate each other, which results in a small offset behavior.
In connection with the present invention, it is to be taken into account that it is known from the state of the art that the error in a frequency (FM) demodulator unit, caused by the group delay time of a filter preceding the frequency (FM) demodulator unit, can be compensated (cf. JP 61-187402). The prior art also discloses several types of frequency (FM) demodulator units (cf. DE-OS 17 66 837, JP-A-04-207209, JP-A-11-055038, JP-A-57-015505, or JP-A-60-070804), as well as filtering of the output signal of a limiter amplifier preceding a frequency (FM) demodulator unit for the purpose of avoiding distortions (cf. JP-A-55-083347).
However, none of the above-mentioned documents known from the state of the art deals with the detection of not only the amplitude butxe2x80x94due to the frequency-dependent transfer characteristicxe2x80x94also the frequency of the input signal. In other words, the prior art documents do not provide any stimulus concerning the reduction of the unwanted influence of the frequency-dependent amplitude variation.
The invention also relates to an integrated circuit, particularly an analog integrated circuit comprising at least one circuit arrangement of the type described hereinbefore and/or operating in accordance with a method as described hereinbefore.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.